1. Field of the Invention
The present invention relates generally to an electromagnetic-based measurement apparatus and method used in well logging. More particularly, the invention relates to antenna structures for such an electromagnetic (EM) measurement apparatus.
2. Background Art
Electromagnetic-based tools or instruments for measuring properties of matter or identifying its composition are well known. For example, resistivity measurements and nuclear magnetic resonance (NMR) measurements are commonly used to infer characteristics of earth formations. The values of electrical conductivity for earth formations have been obtained through the use of EM propagation and induction tools. EM propagation well logging devices are used to measure basic parameters, such as amplitude and phase shift of EM waves that propagate through a medium and, thereby to determine specific properties of the medium.
Electrical conductivity (or its inverse, resistivity) is an important property of subsurface formations in geological surveys and in prospecting for oil, gas, and water because many minerals, and more particularly hydrocarbons, are less conductive than the water filling the pores of sedimentary rocks. Thus, a measure of the conductivity is often a guide to the presence and amount of oil, gas, or water.
EM propagation logging instruments generally use multiple longitudinally-spaced transmitter antennas operating at one or more frequencies and a plurality of longitudinally spaced receiver pairs. An EM wave is propagated from the transmitter antenna into the formation in the vicinity of the borehole and is detected at the receiver antenna(s). A plurality of parameters of interest can be determined by combining the basic measurements of phase and amplitude. Such parameters include the resistivity, dielectric constant, and porosity of the formation, as well as, for example, the degree to which the fluid within the borehole has migrated into the earth formation.
When a time-varying electric current is applied to a transmitter antenna on an induction logging instrument, a time-varying magnetic field is generated. The time-varying magnetic field induces eddy currents in the surrounding earth formations. The eddy currents induce voltage signals in the receiver antennas, which are then measured. The magnitude of the induced voltage signals varies in accordance with the formation properties. In this manner, certain formation properties may be determined.
Conventional antennas used in EM propagation or induction tools consist of coils (or toroids) mounted on the instruments with their axes parallel to the instrument's central or longitudinal axis. Accordingly, the induced magnetic (or electric) field is also substantially parallel to the central axis of the tool and the corresponding induced eddy currents make up loops lying in planes perpendicular to the tool axis.
The response of the described induction logging instrument, when analyzing stratified earth formations, strongly depends on the conductive layers parallel to the eddy currents. Nonconductive layers located within the conductive layers will not contribute substantially to the response signal and therefore their contributions will be masked by the conductive layers' response. Accordingly, in such geometries the nonconductive layers are not accurately detected by typical induction logging instruments.
Many earth formations consist of conductive layers with non-conductive layers interleaved between them. The non-conductive layers contain, for example, hydrocarbons disposed in the particular layer. Thus, conventional lodging instruments are of limited use for the analysis of stratified formations. One way to get past this problem is to use a tool having at least one coil or toroid having its axis tilted or transverse to the longitude axis of the tool.
Solutions have been proposed to detect nonconductive layers located within conductive layers. U.S. Pat. No. 5,781,436 described a method that consists of selectively passing an alternative current through transmitter coils inserted into the well with at least one coil having its axis oriented differently form the axis orientation of the other transmitter coils.
The coil arrangement shown in U.S. Pat. No. 5,781,436 consists of several transmitter coils with their centers distributed at different locations along the instrument and with their axes in different orientations. Several coils have the usual orientation, i.e., with their axes parallel to the instrument axis and, therefore, to the well axis. Others have their axes perpendicular to the instrument axis. This latter arrangement is usually referred to as a transverse coil configuration.
Thus, transverse EM logging tools use antennas whose magnetic or electric moment is transverse to the well's longitudinal axis. The magnetic moment M of a coil or solenoid-type antenna is represented as a vector quantity oriented parallel to the induced magnetic field, that is, perpendicular to the effective plane of the solenoid, with its magnitude proportional to the corresponding magnetic flux. In a first approximation, a coil with a magnetic moment M can be seen as a magnetic dipole antenna. The electric moment P of a toroidal-type antenna is represented as a vector quantity oriented parallel to the induced electric field.
In some applications, it is desirable for a plurality of magnetic or electric moments to have a common intersection, but with different orientations. For example, dipole antennas could be arranged such that their moments point along mutually orthogonal directions. An arrangement of a plurality of dipole antenna wherein the induced moments are oriented orthogonally in three different directions is referred to as a triaxial orthogonal set of dipole antennas.
A logging instrument equipped with an orthogonal set of dipole antennas offers advantages over an arrangement that uses standard dipole antennas distributed at different axial positions along the instrument with their axes in different orientations, such as proposed in U.S. Pat. No. 5,781,436. However, it is not convenient to build orthogonal dipole antennas with conventional solenoid coils or toroids due to the relatively small diameters required for logging instruments. Arrangements consisting of, for example, solenoid coils with their axes perpendicular to the well's central axis occupy a considerable amount of space within the logging instrument.
In addition to the transmitter coils and the receiver coils, it is also generally necessary to equip the induction logging instrument with “bucking” coils in which the magnetic field induces an electric current in the bucking coils that is opposite and equal in magnitude to the current that is induced in the receiver coil when the instrument is disposed within a non-conducting medium such as, for example, air. Bucking coils can be connected in series either to the transmitter or the receiver coil. For a bucking coil with a fixed number of turns, the reciver's output is set to zero by varying the axial distance between the transmitter or receiver coil and the bucking coil. This calibration method is usually known as mutual balancing.
Transverse magnetic fields are also useful for the implementation of NMR based methods. U.S. Pat. No. 5,602,557, for example, describes an arrangement that has a pair of conductor loops, each of which is formed by two saddle-shaped loops lying opposite one another and rotationally offset 90° relative to one another.
An emerging technique in the filed of well logging is the use of tools incorporating tilted coils, i.e., where the coils are tilted with respect to the tool axis. These apparata are configured as such in an effort to alter the direction of the downhole measurement. U.S. Pat. No. 5,508,616 describes an induction tool incorporating tilted transmitter and receiver coils. PCT Application WO 98/00733. Bear et al., describes a logging tool including triaxial transmitter and receiver coils. U.S. Pat. No. 4,319,191 describes a logging tool incorporating transversely aligned transmitter and receiver coils. U.S. Pat. No. 5,115,198 described a tool including a triaxial receiver coil for measuring formation properties. U.S. Pat. No. 5,757,191 describes a method and system for detecting formation properties with a tool including triaxial coils.
The prior art antennas referred to above require winding a coil or toroid. Conventional methods to wind these inductors use wire, but those methods are neither efficient nor reproducible. This is because the tension in the wire and the exact placement of the wire cannot be well controlled. To alleviate those problems, flexible circuit boards have been contemplated for application in a multi-axial antenna design. U.S. Pat. No. 6,690,170 describes copper traces that are mounted on a flexible printed circuit board made of an insulating material to allow the coil or set of coils to be placed on top of an underlying wire-wound axial coil. The transverse saddle coils of the flexible printed circuit board contain four planar rectangular or circular coils of N turns separated from the wire-wound axial coil by the insulating material of the circuit board. When conformed around a non-conducting cylinder, opposite pairs, of planar trances form a transverse coil. One pair has a magnetic dipole moment in the x-direction and the other pair's magnetic dipole moment is in the y-direction. The underlying wire-wound coil is an axial coil having a magnetic dipole moment in the z-direction of the triaxial antenna configuration. These flexible circuit transverse coils have been integral to designing a co-located antenna tool, but do not address the challenges associated with tilted and axial coil or toroid designs.